This invention relates to a method for manufacturing a synthetic resin molding. In particular, it relates to a technique for manufacturing a thick plastic molding using thermal expansion microcapsules.
Plastic moldings have been applied in any field, and have been supplied in the market in large quantity and inexpensively not only as industrial products but also as general consumer products for improving a level of living.
These plastic moldings have a lower melting point than a metal material and their properties can be easily controlled. They are, therefore, formed by a variety of molding methods. Majority of them are prepared by injection molding or extrusion molding. Generally, they are designed to have a minimal required thickness in the light of a size reduction, a weight reduction and a material cost.
On the other hand, some products must be designed to be thicker in view of their functions, performance or design. For example, a propeller fan used in an outdoor unit in an air conditioner is designed to be thick for improved air-blowing performance such as an increase in air volume and reduced noise.
An increase in thickness by simply increasing the amount of a resin, however, leads to increases of weight and material cost as well as an increase of cooling time during molding. Therefore, expansion molding is often employed. Some known expansion molding techniques will be described.
(1) Chemical foaming; a resin mixed or kneaded with a chemical foaming agent such as azodicarboxylic amide is molded.
(2) Nitrogen gas mixing injection molding; a molding machine (screw cylinder) is filled with nitrogen gas during a melting process to inject with a high speed a kneaded resin material into a mold.
(3) Gas assist method; nitrogen gas is introduced from a nozzle in an injection molding machine or a cavity core in a mold during injection filling to form a cavity in a thick part in a molding.
(4) Core back process; a mobile core preformed in a cavity in a mold is positioned in the front during injection, and after filling a resin material, the core is moved backward while feeding a gas, to form a thick part in a cavity structure.
(5) Thermal expansion microcapsule method; microcapsules are mixed with a resin (base resin) and the mixture is foamed in a mold. In this technique, the thermal expansion microcapsules are fine powder with a particle size of about 0.2 to 0.3 mm, which tend to be separated from resin pellets even after being input with the resin pellets into an injection molding machine. Therefore, a master batch in which thermal expansion microcapsules have been kneaded at a high concentration in advance is prepared and then mixed with a base resin (master batch method).
These molding methods described in (1) to (5) have the following drawbacks. A foamed molding prepared using the molding method described in (1) or (2) tends to have a spiral pattern called as a swirl mark on its surface, often leading to problems of product appearance.
The molding methods described in (3) and (4) are ineffective for foaming of a thin part and tend to generate a number of cavities as voids intensely in thick structure parts. Thus, in these techniques, foaming conditions cannot be easily controlled and a lot of skill is required for designing a mold. In addition, it is extremely difficult to keep an even weight balance for each blade in a rotating body such as a propeller fan.
On the other hand, the thermal expansion microcapsule method described in (5) may be advantageous in that the method little generates a swirl mark; eliminates intense formation of voids in a thick structure part; and allows us to conduct expansion molding only by blending a master batch into a base resin. There are, however, still problems to be solved.
Specifically, while a master batch is prepared from thermal expansion microcapsules, heat history causes pre-expansion before molding so that an expansion coefficient during molding a product is lower than an expected value.
According to this invention, an expected expansion coefficient of thermal expansion microcapsules can be satisfactorily achieved during molding a resin in a mold.
Thus, this invention provides a method for manufacturing a synthetic resin molding using thermal expansion microcapsules in which the thermal expansion microcapsules are mixed with a base resin and the mixture undergoes resin molding in a mold, wherein the thermal expansion microcapsules are granulated with a given binder resin under a temperature condition in which the thermal expansion microcapsules are not thermally expanded; then the mixture is mixed with the base resin; and the mixture undergoes resin molding.
According to the method of this invention, the thermal expansion microcapsules are granulated without being exposed to heat history above their thermal expansion temperature, i.e., without pre-expansion, so that an expected expansion coefficient can be achieved during molding a resin in a mold.
Thermal expansion microcapsules contain organic solvent(s) such as a low boiling point liquid hydrocarbon. Thus, the granulation process is preferably conducted at a temperature in a range of 80 to 120xc2x0 C. in the light of avoiding expansion of the solvent and binding action of the binder resin.
For improving miscibility with a base resin with a particle size of about 2 to 3 mm and a length of about 3 to 5 mm, an average particle size of the granulated thermal expansion microcapsules is preferably 7 (particle size: about 2870 mm) to 100 mesh (particle size: about 140 mm). In the granulation process, weatherability additive(s) or pigment(s) may be added as necessary.
A base resin is preferably an olefin resin with a melt flow rate (MFR) of 30 to 90 g/10 min, in order that the resin may be molded at a lower temperature and thermal expansion microcapsules may not be broken during molding.
In molding a resin, a mixture of a base resin and granulated thermal expansion microcapsules may be input into a hopper in an injection molding machine. In such a case, the mixture is preferably input from a vent port in the middle of a cylinder in an injection molding machine for preventing pre-expansion of the thermal expansion microcapsules as much as possible.
In two-material molding, a material to be a core is preferably a recycle resin containing granulated thermal expansion microcapsules, whereby a high-quality product may be provided using more inexpensive materials.